A solar cell is a device that converts photons into electrical energy. The electrical energy produced by the cell is collected through electrical contacts coupled to the semiconductor material, and is routed through interconnections with other photovoltaic cells in a module. The “standard cell” model of a solar cell has a semiconductor material, used to absorb the incoming solar energy and convert it to electrical energy, placed below an anti-reflective coating (ARC) layer, and above a metal backsheet. Electrical contact is typically made to the semiconductor surface with fire-through paste, which is metal paste that is heated such that the paste diffuses through the ARC layer and contacts the surface of the cell. The paste is generally patterned into a set of fingers and bus bars which will then be soldered with ribbon to other cells to create a module. Another type of solar cell has a semiconductor material sandwiched between transparent conductive oxide layers (TCO's), which are then coated with a final layer of conductive paste that is also configured in a finger/bus bar pattern.
In both these types of cells, the metal paste, which is typically silver, works to enable current flow in the horizontal direction (parallel to the cell surface), allowing connections between the solar cells to be made towards the creation of a module. Solar cell metallization is most commonly done by screen printing a silver paste onto the cell, curing the paste, and then soldering ribbon across the screen printed bus bars. However, silver is expensive relative to other components of a solar cell, and can contribute a high percentage of the overall cost.
To reduce silver cost, alternate methods for metallizing solar cells are known in the art. For example, attempts have been made to replace silver with copper, by plating copper directly onto the solar cell. However, a drawback of copper plating is contamination of the cell with copper, which impacts reliability. Plating throughput and yield can also be issues when directly plating onto the cell due to the many steps required for plating, such as depositing seed layers, applying masks, and etching or laser scribing away plated areas to form the desired patterns. Other methods for forming electrical conduits on solar cells include utilizing arrangements of parallel wires or polymeric sheets encasing electrically conductive wires, and laying them onto a cell. However, the use of wire grids presents issues such as undesirable manufacturing costs and high series resistance.
Furthermore, in Babayan et al., U.S. Pat. Nos. 8,569,096 and 8,936,709, which are owned by the assignee of the present application and are incorporated in their entirety by reference herein, electrical conduits for semiconductors such as photovoltaic cells are fabricated as an electroformed free-standing metallic article which are subsequently attached to a semiconductor material. The metallic articles are produced separately from a solar cell and can include multiple elements such as fingers and bus bars that can be transferred stably as a unitary piece and easily aligned to a semiconductor device. The elements of the metallic article are formed integrally with each other in the electroforming process. However, the metallic article is manufactured in an electroforming mandrel, which, while generating a patterned metal layer that is tailored for a solar cell or other semiconductor device, requires additional equipment and cost.
Therefore, there is a need in the industry for low cost methods for attaching electrically conductive elements to the surface of a semiconductor material to thereby form a photovoltaic cell.